Methanol is now noted as a fuel giving rise to less environmental pollution, and methanol engines are therefore being developed now. However, an immediate change-over of fuel used in all kinds of automotive vehicles from gasoline to methanol is almost impossible, and, at the time of change-over, a situation will occur where at least temporarily both the methanol fuel and the gasoline fuel are used.
With a view to deal with such a situation, an introduction of a vehicle is proposed in which both the gasoline fuel and the methanol fuel can be used, that is, a vehicle which has the degree of freedom with respect to fuel to be used. (Such a vehicle will be referred to hereinafter simply as an FFV.)
In order that the engine of such an FFV can be accurately controlled, the blending ratio which is the mixing ratio of fuel between the gasoline and the methanol is to be continuously detected so as to execute various required controls on the engine. A blending ratio detecting device or a sensor for use in such a purpose has been developed and is now in use which can be directly associated with the fuel feed system of the engine so as to directly detect the blending ratio of the fuel.
There is also a method for detecting the blending ratio by the use of an O.sub.2 sensor which generates the information of oxygen concentration in the exhaust gases of an engine, that is, the air/fuel ratio.
However, the practical use of the blending ratio sensor using a conventional photoelectric transducer has had the problems that fouling of its optical system with time must be dealt with, and the sensor cannot operate in its inactive state immediately after the time of engine starting.
On the other hand, in the case of the method using the O.sub.2 sensor, the blending ratio is computed by learning on the basis of the feedback from the O.sub.2 sensor. However, the blending ratio may not be detected as desired since this feedback learning control is not successfully attained until the driving conditions of the vehicle are more or less stabilized.